1. Field of the Invention
The field of the invention is fluidized catalytic cracking of heavy hydrocarbon feeds and cyclones for separating fine solids from vapor streams.
2. Description of Related Art
Catalytic cracking is the backbone of many refineries. It converts heavy feeds into lighter products by catalytically cracking large molecules into smaller molecules. Catalytic cracking operates at low pressures, without hydrogen addition, in contrast to hydrocracking, which operates at high hydrogen partial pressures. Catalytic cracking is inherently safe as it operates with very little oil actually in inventory during the cracking process.
There are two main variants of the catalytic cracking process: moving bed and the far more popular and efficient fluidized bed process.
In the fluidized catalytic cracking (FCC) process, catalyst, having a particle size and color resembling table salt and pepper, circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts a source of hot, regenerated catalyst. The hot catalyst vaporizes and cracks the feed at 425.degree. C.-600.degree. C., usually 460.degree. C.-560.degree. C. The cracking reaction deposits carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper and the stripped catalyst is then regenerated. The catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air. Decoking restores catalyst activity and simultaneously heats the catalyst to, e.g., 500.degree. C.-900.degree. C., usually 600.degree. C.-750.degree. C. This heated catalyst is recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Catalytic cracking is endothermic, it consumes heat. The heat for cracking is supplied at first by the hot regenerated catalyst from the regenerator. Ultimately, it is the feed which supplies the heat needed to crack the feed. Some of the feed deposits as coke on the catalyst, and the burning of this coke generates heat in the regenerator, which is recycled to the reactor in the form of hot catalyst.
Catalytic cracking has undergone progressive development since the 40s. Modern fluid catalytic cracking (FCC) units use zeolite catalysts. Zeolite-containing catalysts work best when coke on the catalyst after regeneration is less than 0.1 wt %, and preferably less than 0.05 wt %.
To regenerate FCC catalyst to this low residual carbon level and to burn CO completely to CO.sub.2 within the regenerator (to conserve heat and reduce air pollution) many FCC operators add a CO combustion promoter. U.S. Pat. Nos. 4,072,600 and 4,093,535, incorporated by reference, teach use of combustion-promoting metals such as Pt, Pd, Ir, Rh, Os, Ru and Re in cracking catalysts in concentrations of 0.01 to 50 ppm, based on total catalyst inventory.
Most FCC's units are all riser cracking units. This is more selective than dense bed cracking. Refiners maximize riser cracking benefits by going to shorter residence times, and higher temperatures. The higher temperatures cause some thermal cracking, which if allowed to continue would eventually convert all the feed to coke and dry gas. Shorter reactor residence times in theory would reduce thermal cracking, but the higher temperatures associated with modern units created the conditions needed to crack thermally the feed. We believed that refiners, in maximizing catalytic conversion of feed and minimizing thermal cracking of feed, resorted to conditions which achieved the desired results in the reactor, but caused other problems which could lead to unplanned shutdowns.
Emergency shutdowns cause substantial economic losses. Modern FCC units must run at high throughput, and for years between shutdowns, to be profitable. Much of the output of the FCC is needed in downstream processing units, and most of a refiners gasoline pool is usually derived from the FCC unit. It is important that the unit operate reliably for years, and accommodate a variety of feeds, including heavy feeds.
The unit must also operate without exceeding local limits on pollutants or particulates. The catalyst is somewhat expensive, and most units have hundred tons of catalyst in inventory. FCC units circulate tons of catalyst per minute, the large circulation being necessary because feed rates are large and for every ton of oil cracked roughly 5 tons of catalyst are needed.
These large amounts of catalyst must be removed from cracked products lest the heavy hydrocarbon products be contaminated with catalyst and fines. Even with several stages of cyclone separation some catalyst and catalyst fines invariably remain with the cracked products. These concentrate in the heaviest product fractions, usually in the Syntower (or main FCC fractionator) bottoms, sometimes called the slurry oil because so much catalyst is present. Refiners frequently let this material sit in a tank to allow more of the entrained catalyst to drop out, producing CSO or clarified slurry oil.
The problems are as severe or worse in the regenerator. In addition to the large amounts of catalyst circulation needed to satisfy the demands of the cracking reactor, there is an additional internal catalyst circulation that must be dealt with. In most bubbling bed catalyst regenerators, an amount of catalyst equal to the entire catalyst inventory will pass through the regenerator cyclones every 15 minutes or so. Most units have several hundred tons of catalyst inventory. Any catalyst not recovered using the regenerator cyclones will remain with the regenerator flue gas, unless an electrostatic precipitator, bag house, or some sort of removal stage is added at considerable cost. The amount of fines in most FCC flue gas streams exiting the regenerator is enough to erode turbine blades if a power recovery system is installed to recover some of the energy in the regenerator flue gas stream. Generally a set of cyclonic separators (known as a third stage separator) is installed upstream of the turbine to reduce the catalyst loading and protect the turbine blades.
While high efficiency third stage cyclones have increased recovery of conventional FCC catalyst from the flue gas leaving the regenerator they have not always reduced catalyst and fines losses to the extent desired. Some refiners were forced to install electrostatic precipitators or some other particulate removal stage downstream of third stage separators to reduce fines emissions.
Many refiners now use high efficiency third stage cyclones to decrease loss of FCC catalyst fines to acceptable levels and/or protect power recovery turbine blades. However, current and future legislation will probably require another removal stage downstream of the third stage cyclones unless significant improvements in efficiency can be achieved.
When a third stage separator is used a fourth stage separator is typically used to process the underflow from the third stage separator. The fourth stage separator is generally a bag house.
Third stage separators typically have 50 or 100 or more small diameter cyclones. One type of third stage separator is described in "Improved hot-gas expanders for cat cracker flue gas" Hydrocarbon Processing, March 1976. The device is fairly large, a 26 foot diameter vessel. Catalyst laden flue gas passes through many swirl tubes. Catalyst is thrown against the tube wall by centrifugal force. Clean gas is withdrawn up via a central gas outlet tube while solids are discharged through two blowdown slots in the base of an outer tube. The device was required to remove most of the 10 micron and larger particles. The unit processed about 550,000 lbs./hour of flue gas containing 300 lbs/hour of catalyst particles ranging from sub-micron to 60 micron sized particles.
We wanted to improve the operation of cyclones, especially their performance on the less than 5 micron particles, which are difficult to remove in conventional cyclones and, to some extent, difficult to remove using electrostatic precipitation.
Based on observations and testing of a horizontal, transparent, positive pressure cyclone, we realized cyclones had a problem handling this 5 micron and smaller size material.
We discovered that turbulent vortices grow along the wall of the cyclones and then shed into the main tangential flow. This caused the particles to hop and bounce away from the wall, reducing collection efficiency.
We wanted to attack the root cause of the problem, and improve the stability of the flow pattern through the cyclone. We discovered that perforations in the body of the cyclone could be used to remove minor amounts of gas with the solids, and have a major impact on stabilizing flow patterns. In addition, by withdrawing some of the gas, and essentially all of the solids, from a plurality of radially distributed openings we eliminated particle reentrainment.